The present invention relates to a method for desulfurizing smoke in which a combustion exhaust gas is cleaned by the use of an absorbing slurry containing limestone particles suspended therein. In particular, the invention relates to a method for desulfurizing smoke by first separating an absorbing slurry, which has absorbed SO.sub.2, into a gypsum slurry and a limestone slurry, returning the limestone slurry to an absorbing process, and recovering gypsum from the gypsum slurry.
FIG. 3 attached hereto shows a schematic view of an apparatus for carrying out a conventional method for desulfurizing smoke.
In FIG. 3, a combustion exhaust gas 202 is cleaned in an absorbing tower 201 of a smoke-desulfurizing apparatus. In an absorbing tower tank 203, an absorbing slurry is stored in which calcium sulfite particles, gypsum particles and limestone particles are suspended. A sulfurous acid gas SO.sub.2 present in the exhaust gas is absorbed in the absorbing slurry by spraying the absorbing slurry from the absorbing tower tank 203 through a spray pipe 205 with the aid of an absorbing tower circulating pump 204. The absorbing slurry sprayed in the exhaust gas becomes a mist state and is then entrained by the exhaust gas. The mist is afterward removed from the exhaust gas by means of a mist eliminator 206 and is then discharged as a cleaned gas 207 from the system. In this mist eliminator 206, the gypsum particles are liable to adhere thereto and to clog it eventually, and thus the mist eliminator 206 is washed by spraying water through a washing nozzle 208.
On the other hand, a limestone slurry which is an absorbing agent is fed to the absorbing tower tank 203 through a limestone feed pipe 209 and is then reacted with SO.sub.2 absorbed in the absorbing slurry to produce calcium sulfite. In this case, the thus produced calcium sulfite is partially oxidized by oxygen contained in the exhaust gas, thereby converting calcium sulfite into gypsum. Further, in order to heighten SO.sub.2 absorbing power of the absorbing slurry, limestone may be fed thereto through the feed pipe 209 in an amount in excess of a level necessary to absorb SO.sub.2. As a consequence, at this point in time, the absorbing slurry stored in the absorbing tower tank 203 is composed of a mixture of the calcium sulfite particles, the gypsum particles, and the limestone particles.
To recover gypsum as a by-product, as described in Japanese Utility Model Provisional Publication No. 10833/1984, the absorbing slurry is drawn out through a valve 211 and is led to a separator 212, in which the absorbing slurry is separated into a gypsum slurry 213 and a mixed slurry 214 containing calcium sulfite and limestone, and the mixed slurry 214 containing calcium sulfite and limestone is then returned to the absorbing tower tank 203 and the gypsum slurry 213 is delivered to a by-product gypsum separator (not shown) via a gypsum slurry tank 215. However, even when the absorbing slurry is separated into the gypsum slurry 213 and the mixed slurry 214 containing calcium sulfite and limestone by means of a centrifugal separator such as a hydrocyclone and a weight sedimentation separator such as a thickener, the result of this separation is insufficient. Accordingly, in the conventional technique, the by-product gypsum is contaminated with calcium sulfite particles and limestone particles, and for this reason, high-purity gypsum cannot be recovered.
The inventors of the present application have investigated the cause of such contamination, and finally it has been found that the ground limestone particles having an average diameter of 10 .mu.m and the calcium sulfite particles having an average diameter of 10 .mu.m which have been produced due to the SO.sub.2 absorbing reaction tend to coagulate in the absorbing slurry, and the coagulated calcium sulfite particles and limestone particles embrace the gypsum particles therein having an average diameter of 40 .mu.m, which fact makes their separation difficult.
In addition, another process for desulfurizing smoke is suggested in Japanese Patent Provision Publication No. 61070/1979. In this process, an absorbing slurry is irradiated with ultrasonic waves to precipitate CaSO.sub.4 which has been dissolved in a supersaturating state in the absorbing slurry, and the precipitated CaSO.sub.4 is then separated and recovered by a thickener. The aqueous alkali solution from which the precipitate has been removed is supplied with Ca(OH).sub.2 and is then circulated through a desulfurizing tower.